Third-party developers of iOS apps today urged iPhone and iPad users not to accept Apple's offer to upgrade iCloud storage to iCloud Drive after they install the new iOS 8.

Although Dropbox was not one of those app makers, it also warned customers about iOS 8 today, telling them a "compatibility issue" would prevent photographs and videos -- but only those files -- from properly uploading and syncing to its service.

But one of the new mobile operating system's touted features may flummox users: iCloud Drive.

Apple unveiled iCloud Drive in June at its annual developers conference, and said it would appear in both iOS 8 and OS X Yosemite, the Mac operating system upgrade slated to ship next month. Seen by some as a return of iDisk, the online file hosting service discontinued in mid-2012, iCloud Drive will let iOS 8 and Yosemite users store files in the ether; allow system-wide access to documents on iOS, liberating those once held in app-specific silos; and store all photographs and videos automatically for sharing and access from any Apple device.

Essentially, iCloud Drive is Apple's response to Dropbox and other services, which show the cloud-based storage and its files in an OS's file manager.

When users upgrade to iOS 8 today and after, they'll be offered to update iCloud's current storage structure to iCloud Drive. (Anyone who has tried OS X Yosemite, either the developer previews or the public beta, has seen the same message.)

Several iOS app developers told their customers not to take Apple up on the iCloud Drive offer, as doing so will make it impossible to synchronize with their apps on devices not running iOS 8, or with Macs running OS X Mavericks or earlier.

Mavericks is the current latest production version of OS X.

"When you upgrade to iOS 8, you need to ensure that you choose the right iCloud Drive option to ensure that Clear for iOS continues to sync with Clear for Mac," said Realmac, the maker of the to-do app Clear, in a blog post today. "If you upgrade to iCloud Drive, you will only be able to sync with devices running iOS 8 or OS X Yosemite."

The same problem will affect any iOS app -- including Apple's, such as the iWork trilogy of Pages, Numbers and Keynote -- which pre-iOS 8 used iCloud for synchronization, document-based or otherwise, between iPhones and iPads, and for syncing to the apps' counterparts on OS X. Once iCloud Drive is switched on in Yosemite or iOS 8, the change cannot be recalled.

Some Computerworld staffers, for example, use Byword, a stark text editor, that comes in versions for both iOS and OS X. Document sync with Byword also failed after iCloud Drive was enabled on a Mac running Yosemite. Although Byword on that Mac successfully saved documents to iCloud Drive, those same documents were unavailable in the Byword app on iOS 7 or even to the OS X version running on a Mavericks-powered Mac.

Realmac and other developers advised customers to decline iCloud Drive by selecting "Not Now" from the pop-up that appears on their screens.

"DO NOT upgrade to iCloud Drive if you want to continue to use iCloud to sync Day One on devices running iOS 7, iOS 8, and OS X Mavericks," warned Bloom Built, the company behind the Day One digital journal and diary, said on its blog today.

Meanwhile, Dropbox alerted its customers of a different problem with iOS 8.

"We've discovered that Apple's new iOS 8 introduces a compatibility issue that may prevent Dropbox and Carousel from properly uploading your photos and videos," the company said on its blog. "This means that only the contents of your 'Recently Added' album will upload automatically." Other files added to Dropbox normally continued to sync.

Dropbox told users to refrain from deleting photos or videos from their devices "until you're sure that your stuff has backed up to Dropbox."

But by 3 p.m. ET, Dropbox said it had fixed the problem and had issued updates to its iOS apps. "We've resolved the iOS 8 compatibility issue. Please update your Dropbox and Carousel apps to get the fix," the firm said on Twitter.

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